Textured pattern sensing using partial-coherence speckle interferometry

ABSTRACT

A system for imaging a textured surface includes a light source that is configured to project an electromagnetic radiation beam onto the textured surface, wherein the projected beam generates first radiation reflected from a first portion of the textured surface to form a speckle pattern, and second radiation reflected from a second portion of the textured surface which is substantially uniform in intensity. The reflected first and second reflected radiation is received by an optical detector, and may be processed to generate an image of the textured surface from the first and second reflected radiation. Methods for textured surface sensing are also disclosed.

BACKGROUND

This application generally relates to textured pattern sensing anddetection, and more particularly, sensing and detecting usingpartial-coherence speckle interferometry.

A need exists in the fields of counterterrorism and law enforcement toidentify and track suspected terrorists or felons from a distance,without the suspect's knowledge or cooperation, and without leaving atrail that might alert the suspect that he is under suspicion, and bywhom. A number of biometric sensing and tracking concepts have beenproposed. For instance, remote fingerprinting has been identified as anattractive means for identifying and tracking of terrorists.

A problem, however, in remote, covert fingerprinting of a suspectedterrorist or felon lies in the lack of contrast in detectingcharacteristics of the fingerprint. For instance, there may beinsufficient differentiation in the reflectivity, emissivity, orpolarization signature between the high points (or ridges) and the lowpoints (or valleys) of the dermal papillae to meet needs under a broadrange of conditions.

This problem results because passive sensors typically require someintensity, spectral, polarimetric, or other form of image contrast todifferentiate ridges from valleys in the dermal papillae. The subtlenatural differentiation based on passive signatures may proveinsufficient for discrimination, except under very unusual conditions(e.g., shallow illumination grazing angles adequate to produce shadows).Systems exist that create unnatural differentiation by selectivelyadding an artificial pigment to either the ridges or valleys. In fact,this is how traditional ink-on-paper fingerprints are taken.

Commercial 3-D Laser Detection and Ranging (LADAR) sensors based ondirect detection pulse-echo ranging techniques are presently used fordetection variety of 3-dimensional imaging applications. However, theytypically do not have the range resolution necessary to measure thesubmillimeter height difference between ridges and valleys in thetexture pattern of a human finger. Moreover, commercial LADAR sensorswhich can record fingerprints using range contrast require that thesubject place his/her finger on a flat glass optical surface or windowwhich provides a flat datum for a “binary” range determination as towhether the texture feature is at the same range as the datum or not.The binary approach can provide sharp fingerprint detail, but requiresthe suspect to put his/her hand on a flat optical surface or window forscanning and is therefore not effective for covert, remote fingerprintcapture.

Laser-based interferometric approaches to high resolution profilometrymight also be considered. These require the use of a coherent lightsource to interrogate the target and the return signal is opticallymixed with a coherent local oscillator signal of the same wavelength ina heterodyne detection process. When the return signal and localoscillator signal are in phase, the mixed signal is strong, due toconstructive interference. When the return and local oscillator signalsare out of phase, the mixed signal is measurably weaker due todestructive interference. Regions of constructive interference in theinterferometric image of the finger appear as higher intensity fringesand are separated by the weaker intensity regions of destructiveinterference. The difference in height from one fringe to the next isprecisely the wavelength of the optical signal. This fringe patterntherefore provides a very accurate measure of 3-D surface features, muchlike a topographic map, where the fringe lines are equivalent to linesof constant surface elevation. This interferometric approach works wellwhen the surface to be profiled is relatively flat so that the fringelines are separated by more than a pixel and can be distinguished in theimage. Unfortunately, this is not the case with fingers when using alight source in the ultraviolet, visible, or infrared regions of thespectrum.

As a result, these conventional approaches have been generallyimpractical in real-time terrorist-identification scenarios whereequipment cannot be pre-positioned, the range is quite variable, and/orthe fingerprint must be taken remotely and covertly. The rangevariability may arise from several factors: the distance between thesensor and the suspect's fingers is not precisely known, the dermalpapillae are on a quasi-cylindrical surface and therefore have depth,and/or the target is in motion.

SUMMARY

In an embodiment, a system for imaging a textured surface comprises: alight source configured to project electromagnetic radiation onto thetextured surface, wherein the projected beam generates first radiationreflected from a first portion of the textured surface to form a specklepattern, and second radiation reflected from a second portion of thetextured surface which is substantially uniform in intensity; and anoptical detector configured to receive the first and second reflectedradiation from the textured surface.

In another embodiment, a method for imaging a textured surfacecomprises: projecting a beam of electromagnetic radiation onto thetextured surface, wherein the projected beam generates first radiationreflected from a first portion of the textured surface to form a specklepattern, and second radiation reflected from as a second portion of thetextured surface which is substantially uniform in intensity; andreceiving, with an optical detector, the first and second reflectedradiation from the textured surface.

These and other aspects of this disclosure, as well as the methods ofoperation and functions of the related elements of structure and thecombination of parts and economies of manufacture, will become moreapparent upon consideration of the following description and theappended claims with reference to the accompanying drawings, all ofwhich form a part of this specification, wherein like reference numeralsdesignate corresponding parts in the various figures. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration and description only and are not a limitation of theinvention. In addition, it should be appreciated that structuralfeatures shown or described in any one embodiment herein can be used inother embodiments as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a photograph of the cross-section of a human fingerincluding the physiology of the dermal layers, and FIG. 1 b shows amodel of a typical human fingerprint;

FIG. 2 depicts a block diagram of an exemplary textured pattern sensingsystem in accordance with an embodiment;

FIG. 3 shows a photograph of one detailed speckle pattern of anexemplary fingerprint;

FIG. 4 shows an exemplary method for textured pattern sensing inaccordance with an embodiment.

FIG. 5 shows key parameters of a model for fingerprint topography thatare used to estimate the coherence length of a partially-coherent lasertransmitter in accordance with an embodiment.

FIGS. 6 a-6 d show processed images of a typical fingerprint inaccordance with an embodiment;

FIG. 7 shows a block diagram of an another exemplary textured patternsensing system in accordance with an embodiment;

FIG. 8 depicts another exemplary method for textured pattern sensing inaccordance with an embodiment; and

FIGS. 9 a-9 d show processed images of a typical fingerprint inaccordance with an embodiment.

DETAILED DESCRIPTION

A methodology for textured pattern sensing and detection is disclosed.According to an embodiment, a light source is configured to project anelectromagnetic radiation beam onto a textured surface, wherein theprojected beam generates first radiation reflected from a first portionof the textured surface to form a highly modulated speckle pattern, andsecond radiation reflected from a second portion of the textured surfacewhich is substantially uniform in intensity. The reflected first andsecond reflected radiation is received by an optical device, which issubsequently processed by a processor to generate an image of thetextured surface from the first and second reflected radiation.

A speckle pattern is a random intensity pattern that may be produced bythe mutual interference of plural waves having different phases whichadd together to give a resultant wave whose amplitude and intensity varyrandomly. For instance, when a textured surface is illuminated by acoherent light wave, each point thereof acts as a source of secondarywaves, and light at any point in the scattered light field is a coherentsuperposition of waves which have been scattered from each point on theilluminated surface. The amplitude of the resultant electric field isthe sum of the electric field amplitudes of these secondary waves andcan range from some maximum due to complete constructive interference tozero due to complete destructive interference. Thus, the surfacetopography results in scattered reflected light randomly varying inintensity.

If the light is partially-coherent, its optical frequency (andwavelength) will not have a single value but will be spread over afinite range, which we assume to have a Gaussian line shape. Suchpartially-coherent light may be characterized by a coherence length,l_(C), defined as the optical path length difference at which the fringevisibility of a light beam, which is interfered with itself, is reducedto 2^(−1/2). By this definition, the coherence length is equal to 0.64c/δν, where c is the speed of light in vacuum and δν is the spread inoptical frequency between the half power points of the presumed Gaussianline shape. For partially-coherent light, the interference betweenregions of the surface, which differ in range by more than l_(c), willbe substantially less sharp and the resultant intensity variations willbe effectively averaged out.

While reference herein is made to fingerprints, it will be appreciatedthat the disclosed methodology may be similarly used for detecting othertextured dermal surfaces, such as toeprints, footprints, palmprints,etc. Other textured patterns or surfaces may similarly be detected, suchas, for example, facial features, woven clothing patterns, and grainpatterns of an item, such as leather, wood, metal, ceramic, plastic orother items.

FIG. 1 a shows a photograph of the cross-section of human finger 50including the physiology of the dermal layers. Finger 50 includesepidermis 10 and dermis 20 layers of skin. Epidermis 10 is the outerlayer of skin and includes the papillary ridges 12, papillary valleys14, and pores 16. Dermis 20 is located between epidermis 10 and fatcells 30 and includes papilla 22, tactile corpuscles and nerves 24,sweat duct and gland 26, and blood vessels 28. The configuration ofpapillary ridges 12 and valleys 14 is different for each person, andthus creates a unique fingerprint for each person.

FIG. 1 b shows a model of human fingerprint 100. The topography of humanfingerprints based on laser profilometry has been reported, for example,by M. Stucker, et al., “Interpapillary Lines—The Variable Part of theHuman Fingerprint,” Journal of Forensic Sciences, Vol 46, Issue 4, (July2001). Their data indicates that human fingerprints have an averagepapillary ridge height: of 0.0590±0.0192 mm and papillary ridge spacing:0.4355±0.0574 mm.

The median width of ridges 120, and valleys 140, may be about 0.276 mmand 0.160 mm, respectably. The convolutions defining papillary ridges120 and papillary valleys 140 are generally not symmetric. Ridges 120are typically “U” shaped and broader than valleys 140, while the foldsin valleys 140 are substantially “V” shaped. This “V” shaped featureleads to partial trapping of incident electromagnetic radiation (light)in valleys and a contrast reduction in valley region 140 relative to theridge region 120.

FIG. 2 shows an exemplary textured pattern sensing system 200 forremote, covert fingerprint recording, according to an embodiment. Targetfinger 50 of a suspect includes a plurality of papilla ridges 12 andvalleys 14 which define a unique fingerprint 100. Because a typicalfinger appears as a diffuse reflector of visible light, the dermalsurface has an inner-scale roughness on the order of a micrometer.

Laser driver 215 controls laser transmitter 220 which is configured toprovide illumination of target finger 50. In one implementation, lasertransmitter 220 may be a partially coherent pulsed solid-state lasertransmitter that operates in the near infrared portion of theelectromagnetic radiation spectrum beyond the normal photopic responserange of the human eye (e.g., greater than 770 nm), but within thespectral response of typical sold-state optical detectors, e.g., siliconcharge coupled devices (CODs). For instance, a low-average power laseroperating in the visible band may also be undetectable if the ambientlight is sufficiently high (e.g., daylight conditions).

Of course, it will be appreciated that other light sources may also beused, such as, laser diodes, light emitting diodes, continuous wave (CW)solid-state lasers, filtered incandescent lamps, atomic vapor lamps,fluorescent tubes, etc. And it will be appreciated that various spectraof light could be used in accordance with the embodiments disclosedherein, such as visible, ultraviolet (UV), or infrared (IR).

Timing and control electronics 210 direct laser driver 215 and lasertransmitter 220 to fire and manage the timing of the bias and readoutcycles for the optical detector 235. Such systems are known in the artfor controlled laser pulse timing. Separate mode control 205 may be usedto switch the timing of the laser driver 215 and the frame rate and orformat of the optical detector 235 based on the target size, range,reflectivity, surface texture characteristics, atmospheric transmission,and ambient lighting conditions. For example, a dark target surface mayrequire greater pulse energy or a longer dwell time (CW source) toachieve the desired signal-to-noise ratio relative to a lighter targetsurface. If the laser is a continuously-pumped and Q-switched solidstate laser, the Mode Control 205 may command the Timing and ControlElectronics 210 to lengthen the period between laser pulses resulting ina higher laser pulse energy and also to reduce the frame rate of theoptical detector to maintain synchronization with the new laser pulserate.

Transmit optics 225 may be provided and configured to shape, focus,and/or correct aberrations of the beam of electromagnetic radiation(light) 226 and direct it towards target finger 50. Receiver optics 230may include one or more lens, polarizers, filters, etc., as known in theart. The transmit optics 225 and receive optics 230 functions may becombined within a single optical system which may have shared elementssuch as lenses.

Speckle pattern 228 resulting from scattering the laser beam from thetextured surface, such as fingerprint 100, is collected by receiveroptics 230 which image speckle pattern 228 onto optical detector 235.Receiver optics 230 may include a narrow bandpass optical filter tominimize reflected solar or thermal background noise. Speckle pattern228 is sensed by optical detector 235 as an intensity image. In someimplementations, optical detector 235 may be a charge coupled device(CCD), photodiode array, camera or other detector sensitive toelectromagnetic radiation or light.

The speckle pattern imaged by the optical detector 235 may then becaptured by the frame grabber memory 245 or sent directly to processor250 which extracts features, e.g., corresponding to papillary ridges andvalleys of target finger 50. Processor 250 may also read instructionsfor an algorithm known in the art of forensic and biometric sciences toextract higher-level attributes such as fingerprint class (e.g., whorl,right loop, left loop, arch, and tented arch) and features such asminutiae points (e.g., local ridge characteristics that occur at eithera ridge bifurcation of ridge ending) or correlation registration points.The output of processor 250 may include standard video interface format(such as, for example, IEEE RS 170), or a standard computer-interfaceformat (serial or parallel).

The pointing function of textured pattern sensing system 200 may becontrolled by using a separate imager and video tracker system (notshown) which may be configured to point the sensor line-of-sight (LOS)toward target finger 50. This pointing function might also beaccomplished by the human operator using a small display, such as, forexample, a liquid crystal display (LCD) device. With the imager, videotracker, and LOS pointing means, the field-of-view (FOV) of texturedpattern sensing system 200 may need only be the size of the targetsuspect's palm.

Rangefinder 255 may also be included, which drives zoom, iris and focuscontrol servo mechanism 260 attached to receiver optics 230 to (1)maintain fingerprint image 100 in sharp focus on optical detector 235,(2) maintain a near constant pixel spacing between papillary ridges 12,and/or (3) maintain a near constant ratio between the diameter of thereceiver point spread function and the size of papillary features in theimage plane of the optical detector 235. To maintain focus, the accuracyof rangefinder 255 may be comparable to the depth-of-focus of receiveroptics 230. In some implementations, range finder 255 may use a pulsefrom laser transmitter 220, a fast photodiode (not shown), and aconventional range counter to measure the time of flight of the laserpulse in the round trip to and from the target and use this to calculatethe range to the target and the corresponding focus position of thereceive optics. Alternatively, a separate laser rangefinder, acousticrangefinder, image-based system designed to peak spatial frequency inthe image, or other auto-focusing means may be used separately, or inconjunction with the pulse-echo laser rangefinder described above.

The ranging requirement for focus control may be substantially moredemanding than for zoom and iris control.

Instructions for a model based algorithm used to discriminate the ridgesfrom valleys may reside in processor 250, or in a separatecomputer-readable physical storage medium. Fingerprint image 270 may beoutput from processor 250. Output image 270 may be stored in memory (ordatabase), displayed, printed, etc. Depending on the needs of lawenforcement or government personnel, fingerprint image 270 may be runthrough a database of fingerprint images, to identify suspects and/orterrorists.

FIG. 3 shows a photograph of detailed speckle pattern 300 of afingerprint. As shown, speckle pattern 300 includes a random intensitypattern produced by the mutual interference of a set of wavefronts.Regions of high speckle granularity correspond to the ridges, where thedepth across a pixel is less than the coherence length of the opticalsource. Regions of low intensity variation correspond to the valleys,where the depth across a pixel is greater than the coherence length ofthe source and where the speckle features are averaged. In oneimplementation, speckle pattern 300 was formed using a QPhotonicQSDM-915 laser diode manufactured by Laser Diodeaser, Inc., as the lasersource. This laser was configured to have a center wavelength of about909 nm, a spectral width of about 44 nm, and a coherence length of about8.3 μm. The receiver optics were configured to have a focal length ofabout 75 mm, a working distance of about 180 mm, a variable aperture,and a depth of focus of about 3.5 mm. This yielded a resolution ofapproximately 18.5 pixels between adjacent ridges 12.

FIG. 4 shows an exemplary method 400 for detecting a textured pattern inaccordance with an embodiment.

Electromagnetic radiation 405 emitted, for instance, from a lasertransmitter 220 (FIG. 2) may be directed towards a textured pattern fromwhich it is reflected. Laser transmitter 220 may emit partially-coherentlaser light of a specific coherence length. Note, for ease of clarity,radiation absorbed by finger 50 and/or reflected in directions otherthan toward detector surface 435 has been omitted.

In the embodiment shown, the textured pattern is of fingerprint 100 offinger 50, and includes a unique configuration of papilla ridges 12 andvalleys 14 which define an individual's fingerprint. Of course, othertextured patterns may be similarly detected. Region 435 shows a portionof fingerprint 100 imaged on focal plane of optical detector 235 (FIG.2).

When textured pattern sensing system 200 is trained on a target finger50, each relatively flat papillary ridge 12 will reflect radiation 410that will register as a sharp speckle pattern 415, with a granularity ofthe order of the optical detector's diffraction-limited resolution(i.e., approximately equal to the laser wavelength divided by thereceiver aperture diameter in units of radians). On the other hand, eachvalley region 14 between papillary ridges 12 will reflect radiation 420that will register as being substantially uniform region 425 (i.e.,constant mid-level intensity, with no distinguishing features, due tospeckle averaging). This speckle averaging in valley regions 14 occursbecause the physical depth of the sloped valleys region 14, as imagedwithin a single pixel, is greater than the coherence length of apartially-coherent laser transmitter beam. An algorithm may be used todiscriminate the relatively flat ridge regions form the slope valleyregions using processor 250. The algorithm may be a pattern recognitionalgorithm.

Fingerprint image 450 may be output from processor 250. Ridges 12 appearas “lighter” areas, and valleys 14 appear as “darker” areas. Thecontrast between the darker and lighter areas forms an image of thetextured surface. In this case, the image detected corresponds tofingerprint 100.

One specification established for the scanning resolution for exchangeof fingerprint information specifies 19.96 pixels per millimeter (ppmm).See “American National Standard for Information Systems—Data Format forthe Exchange of Fingerprint, Facial, and Scar Mark & Tatoo (SMT)Information,” ANSI/NIST-ITL 1-2000, p. 5, 2000. This corresponds toapproximately a 0.050 mm pixel spacing, or an average of about 8.7pixels between adjacent ridges 12. A slightly higher resolutioncorresponding of about 12 pixels between adjacent ridges 12 may bedesirable (or 0.0363 mm pixel spacing) to provide good speckle contrast.This may provide 4 full pixels across each valley 14, to sufficientlydefine the speckle-average region clearly for the pattern recognitionalgorithm. For a normally incident finger and a sensor-to-targetdistance of about 3 meters, the desired angular resolution of the sensormay be about 12.1 microradians.

Achieving this resolution, the pixel size is matched to the specklefeature size (λ/D). In one implementation, this may be achieved using agallium arsenide (GaAs) diode laser having a wavelength of 904 nm andreceiver optics of approximately 7.5 cm (2.9 inch) diameter. This issufficiently small for clandestine surveillance.

FIG. 5 shows key parameters of model 500 for fingerprint topography thatare used to estimate the coherence length of the partially-coherentlaser transmitter in accordance with an embodiment.

Adjacent valleys 12 are assumed to be spaced apart by a distance s, witha depth d from surface ridges 14. Assuming that the local topography ofthe fingerprint is hyper-sinusoidal, the height (h) at any point (x)across the cross-section of the dermal papillae may be characterizedaccording to equation (1) as follows:

h=d(1−sin¹⁰(πx/s))  (1)

This equation may be a good fit to the outer scale cross-sectional datain FIG. 1 a. The inner-scale roughness of the finger surface, which isresponsible for its diffuse reflection characteristic, may be modeled byadding a random variation in height with a root mean square (RMS)amplitude of 1 micrometer. Exemplary parameters illustrated in FIG. 5are as follows:

d=depth of valley=0.059 mm

s=spacing between valleys=0.436 mm

λ=wavelength=904 nm

a=pixel size<<s (let a=s/12=0.0363 mm)

h₁=minimum height variation across pixel at edge of valley<l_(c)

h₂=minimum height variation across pixel at middle of valley>l_(c)

l_(c)=laser coherence length (m)=0.64 c/δν

c=speed of light=3×10⁸ m/s

δν=laser linewidth (Hz)

The laser coherence length may be selected through a parametrictradeoff. For instance, a design or simulation tool, such as MATLAB®software may be used for optimizing one or more parameters. Using theabove parameter values, the minimum depth of the surface texture withinthe 4 pixels nearest the valley under worst case conditions regardingpixel position or shift is as follows:

h_(1MIN)=h_(2MIN)=0.234 d=0.014 mm

l_(c)=0.014 mm

δν=0.64 c/l_(c)=13.9 THz

ν=331 THz (for 904 nm laser wavelength)

Δλ=λ(δν/ν)=37.9 nm

Under these conditions, the approximate depth-of-field may be about 3.54mm, which sets the precision of the rangefinder function. This assumes acircle of confusion of ½ pixel or 0.0182 mm, and a typical pixel pitchof the camera FPA of 0.015 mm. To achieve this precision using thepartially-coherent laser transmitter as the rangefinder source in apulse-echo rangefinder configuration (not shown) may require a pulsewidth on the order of about 180 picoseconds, very high speed constantfraction discrimination circuit (or equivalent) and high speed rangecounting electronics. It may, therefore, be more advantageous to use aseparate coherent laser rangefinder or other means known in the art ofcamera autofocus control (e.g., acoustic rangefinder or image-basedautofocus system designed to peak the spatial frequency in the image),or a combination thereof for this focus control function. A zoomcapability may also be desired to maintain a near constant pixel spacingbetween papillary ridges. However, the ranging requirement for focuscontrol will be substantially more demanding than for zoom control. Thephysical effective focal length may be accommodated with a telephotolens system to minimize the physical size of the sensor.

FIGS. 6 a-6 d show images of a simulated fingerprint according to anembodiment.

FIG. 6 a shows a textured pattern used by a MATLAB® script to simulatethe three-dimensional textured surface of a typical finger, whereinintensity represents the height of the surface. The outer-scale ridgesand valleys are modeled as a hypersinusoid and the inner-scale roughnessis modeled by adding a random height variation with a 1 micrometer RMSamplitude. The high points in the pattern (ridges) are black in thefigure; and the low points (valleys) are white. A scale is shown at theright of FIG. 6 a, the full range for which is 0.06 mm. The MATLAB®script was also used to simulate the functionality of textured patternsensing system 200.

FIG. 6 b shows the MATLAB® simulation of the speckle image as may bedetected by optical detector 235 of textured pattern sensing system 200corresponding to the simulated fingerprint texture pattern of FIG. 6 a.For this simulation, the center wavelength of laser transmitter 220 is904 nm and the spread in transmitter operating wavelengths is modeled byusing 40 discrete transmitter lines weighted with a Gaussian amplitudeprofile with a line width of 166 nm at the half power points,corresponding to a spectral separation of 60.9 THz. The coherence lengthis given by l_(c)=0.64 c/δν which is 3.15 micrometers. This MATLAB®program models the phase of the received electromagnetic radiationwithin each pixel as the average of the phases at the center pointwithin each sub-element of a 4×4 element array used to model that pixel,therefore the aperture averaging is less complete than would be the casefor a contiguous pixel within optical detector 235, and the heightvariation across the pixel is only half the height variation across a“real” pixel. For this reason, the MATLAB® program under-predicts thecoherence length required to provide good contrast between ridge andvalley features (0.00315 mm simulated vs. 0.014 mm calculated).

FIG. 6 c shows the result of high/low threshold enhancement of thespeckle image where all pixels having an intensity value between 0.8 and5.0 times the standard deviation in pixel intensity are set to zerointensity (white in figure) and all pixels having intensity valuesoutside this range are set to maximum intensity (black). This processlargely removes the speckle averaged values associated with thepapillary valley features of target finger 100. FIG. 6 d shows theresult of point-expansion enhancement of the thresholded image of FIG. 6c where all pixels in a five pixel radius surrounding each pixel with amaximum intensity value is also set to maximum intensity. This processfurther enhances the ridge features by more completely filling in thesefeatures without significantly changing the valley features. Thethreshold and point expansion processes shown in FIGS. 6 c and 6 d aswell as other image enhancement processes may be performed by processor250 of texture pattern sensing system 200.

FIG. 7 is a block diagram of a texture pattern sensing system 700 thatused two-partially-coherent laser transmitters with frame subtraction,in accordance with another embodiment.

System 700 may be configured for remote, covert fingerprint detectingsimilar to system 200 (FIG. 2). In this embodiment, two lasertransmitters 720 a, 720 b may be used as the illumination source fortarget finger 50, the return signals from each 728 a, 728 b are imagedusing two optical detectors 735 a, 735 b, each connected to a framegrabber memory 745 a, 745 b. All other related elements may be similarlyconfigured as shown in system 200 (FIG. 2).

For instance, laser driver 715 provides input to laser transmitters 720a, 720 b which are configured to provide illumination source for targetfinger 50. In one implementation, laser transmitters 720 a, 720 b may bea partially-coherent pulsed solid-state laser transmitter which operatesin the near infrared wavelength spectra. Although, it will beappreciated that various spectra could be used in accordance with theembodiments disclosed herein, such as visible, ultraviolet (UV), orinfrared (IR). The separation in operating frequency between lasertransmitters 720 a, 720 b is selected such that the speckle patternsassociated with the relatively flat ridge regions are not wellcorrelated. The temporal width of the laser pulse should be sufficientto “freeze” the motion of system 700 and target finger 50. A pulse, forinstance, having a width shorter than about 100 μs should be sufficientfor most remote, covert fingerprinting scenarios. Transmit optics 725may be provided that are configured to shape, focus, and/or correctaberrations of the beam of electromagnetic radiation (light) 726 a, 726b from respective laser transmitters 720 a, 720 b and direct themtowards target finger 50.

Speckle patterns 728 a, 728 b resulting from scattering thepartially-coherent laser transmitter beam from the textured surface,such as fingerprint 100, are collected by receiver optics 730 whichimages speckle patterns 728 a, 728 b onto respective optical detectors735 a, 735 b. Receiver optics 730 may include a narrow bandpass opticalfilter to minimize reflected solar background noise. Speckle patterns728 a, 728 b are imaged by optical detector 735 a, 735 b. In someimplementations, optical detectors 735 a, 735 b may be a charge-coupleddevice (CCD), photodiode array, camera or other detector.

The output of frame grabbers 745 a, 745 b may be sent to one or moreprocessors 750 which may be configured to perform image processing.Frame grabber 745 is configured to capture individual images generatedby the optical detector 735. In some embodiments, the speckle patternimaged by optical detector 735 may be sent directly to processor 750which is configured to extract features (e.g., papillary ridges andvalleys of the target finger) as a fingerprint image.

Processor 750 may utilize one or more algorithms, including those knownin the art of forensic and biometric sciences, to extract higher-levelattributes from a fingerprint. Rangefinder 755 may also be included,which drives zoom, iris and focus control servo mechanism 760 attachedto receiver optics 730 to (1) maintain fingerprint image 700 in sharpfocus on optical detectors 735 a, 735 b, (2) maintain a near constantpixel spacing between papillary ridges 12, and/or (3) maintain a nearconstant ratio between the diameter of the receiver point spreadfunction and the size of papillary features in the image plane of theoptical detectors 735 a, 735 b. To maintain focus, the accuracy ofrangefinder 755 may be comparable to the depth-of-focus of receiveroptics 730. In some implementations, range finder 755 may use a pulsefrom laser transmitter 720, a fast photodiode (not shown), and aconventional range counter to measure the time of flight of the laserpulse in the round trip to and from the target and use this to calculatethe range to the target and the corresponding focus position of thereceive optics. Alternatively, a separate laser rangefinder, acousticrangefinder, image-based system designed to peak spatial frequency inthe image, or other auto-focusing means may be used separately, or inconjunction with the pulse-echo laser rangefinder described above.

Timing and control electronics 710 direct laser driver 715 and lasertransmitter 720 to fire and manage the timing of the bias and readoutcycles for the optical detectors 735 a, 735 b. Such systems are known inthe art for controlled laser pulse timing. Separate mode control 705 maybe used to switch the timing of the laser driver 715 and the frame rateand or format of the optical detectors 735 a, 735 b based on the targetsize, range, reflectivity, surface texture characteristics, atmospherictransmission, and ambient lighting conditions. For example, a darktarget surface may require greater pulse energy or a longer dwell time(CW source) to achieve the desired signal-to-noise ratio relative to alighter target surface. If the laser is a continuously-pumped andQ-switched solid state laser, the Mode Control 705 may command theTiming and Control Electronics 710 to lengthen the period between laserpulses resulting in a higher laser pulse energy and also to reduce theframe rate of the optical detector to maintain synchronization with thenew laser pulse rate.

The pointing function of textured pattern sensing system 700 may becontrolled by using a separate imager and video tracker system (notshown) which may be configured to point the sensor line-of-sight (LOS)toward target finger 50. This pointing function might also beaccomplished by the human operator using a small display, such as, forexample, a liquid crystal display (LCD) device. With the imager, videotracker, and LOS pointing means, the field-of-view (FOV) of texturedpattern sensing system 200 may need only be the size of the targetsuspect's palm.

The ranging requirement for focus control may be substantially moredemanding than for zoom and iris control. Instructions for a model basedalgorithm used to discriminate the ridges from valleys may reside inprocessor 750, or in a separate computer-readable physical storagemedium. Fingerprint image 770 may be output from processor 750. Outputimage 770 may be stored in memory (or database), displayed, printed,etc. Depending on the needs of law enforcement or government personnel,fingerprint image 770 may be run through a database of fingerprintimages, to identify suspects and/or terrorists.

A frame subtraction function may be performed by processor 750 whereinthe image stored in frame grabber memory 745 a is subtracted from theimage of that stored in frame grabber memory 745 b, resulting in asingle frame of imagery. Additional processing may be performed tolevel-balance the two images before subtraction, such that the intensityin the valley regions of the image in 745 a is approximately equal tothat in 745 b. The result is that the speckle-averaged regionscorresponding to the valley features will essentially vanish in thesubtracted frame, leaving a highly detectable speckle pattern at theridge features that is easier to process.

FIG. 8 shows textured pattern sensing method 800 using twopartially-coherent laser transmitters with frame subtraction inaccordance with an embodiment.

When system 700 is trained on target finger 50, each relatively flatpapillary ridge 12 will register as a sharp speckle pattern and eachvalley 14 will register as a substantially uniform intensity.

If the center wavelengths of the two partially-coherent lasertransmitters are sufficiently separated, the speckle patterns will beuncorrelated between the two optical detectors' images, and thesubtracted frame will show granular intensity patterns similar to theoriginal speckle patterns, but with some higher spatial frequencycomponents. The valley regions will subtract to a near-zero intensity,provided the two camera images are properly corrected for pixel-to-pixelnonuniformity and overall intensity balance. This subtraction processmay provide improved contrast, making the image enhancement and featureextraction more robust.

Electromagnetic radiation 805 a, 805 b emitted, for instance, frompartially-coherent laser transmitters 720 a, 720 b (FIG. 7), may bedirected towards a textured pattern, in this case fingerprint 100, whichis reflected. Note, for ease of clarity, radiation absorbed by finger 50and/or reflected in directions other than toward detector surfaces 735a, 735 b has been omitted.

In the embodiment shown, the textured pattern is fingerprint 100 offinger 50, including a unique configuration of papilla ridges 12 andvalleys 14 which define an individual's fingerprint. Of course, othertextured patterns may be similarly detected. Regions 835 a, 835 b showportions of fingerprint 100 imaged on focal plane of optical detectors735 a, 735 b, (FIG. 7), respectively.

When the textured pattern sensing system is trained on a target finger50, each relatively flat papillary ridge 12 will reflect radiation 810a, 810 b, which will register as sharp speckle patterns 815 a, 815 b,with a granularity on the order of the optical detector'sdiffraction-limited resolution (i.e., approximately equal to the laserwavelength divided by the receiver aperture and quantified in units ofradians). On the other hand, each transition valley region 14 betweenpapillary ridges 12 will reflect radiation 820 a, 820 b which willregister as being substantially uniform regions 825 a, 825 b (i.e.,constant mid-level intensity with no distinguishing features due tospeckle averaging).

The intensity patterns of the resulting images produced from reflectedradiation 810 a, 810 b may be subtracted from reflected radiation 820 a,820 b, respectively, and the absolute value taken, as shown at 840,resulting in fingerprint image 850. This subtraction process willproduce a high contrast between the ridge and valley regions if threeconditions are met. First, the physical depth of the sloped valleysregions 14 of fingerprint 100, as imaged within a single pixel, isgreater than the coherence length of each partially-coherent lasertransmitter 220 a, 220 b. Second, physical depth of the relatively-flatridge region 12, as imaged within a single pixel, is less than thecoherence length of each partially-coherent laser transmitter beam.Third, the separation in center operating frequency between the twopartially-coherent laser transmitters is sufficiently large as given bythe following equation, where ν₁ and ν₂ are the center frequencies ofthe transmitters, c is the speed of light, and d is the texture depth ofthe relatively flat ridge region. Given a texture depth of about 1micrometer, the frequency separation should be greater than 37.5 THz.This is equivalent to a wavelength separation of 102 nm at an operatingwavelength of 904 nm.

ν₂−ν₁ >c/(8d)

An algorithm may be used to discriminate the relatively flat ridgeregions from the slope valley regions used by processor 750. Thealgorithm may be a pattern recognition algorithm.

FIGS. 9 a-9 d show images of a typical target finger derived fromMATLAB® simulation of the second embodiment. FIG. 9 a shows the sametextured pattern as FIG. 6 a used by a MATLAB® script to simulate thethree-dimensional textured surface of a typical finger. FIG. 9 b showsthe same MATLAB® simulation of the speckle image as FIG. 6 b as may bedetected by optical detector 735 a. FIG. 9 c shows a MATLAB® simulationof the speckle image as may be detected by optical detector 735 b. Forthis simulation, the center wavelength of laser transmitter 720 b is 631nm and the spread in transmitter operating wavelengths is modeled byusing 40 discrete transmitter lines weighted with a Gaussian amplitudeprofile with a line width of 80.8 nm at the half power points,corresponding to the same spectral separation of 60.9 THz and the samecoherence length of 3.15 micrometers.

FIG. 9 d shows a processed image based on subtracting normalized pixelintensity values within the individual speckle images detected byoptical detectors 735 a, 735 b of texture pattern sensing system 700then taking the absolute value of the resultant pixel values using aMATLAB® script. The ridge features shown in FIG. 9 d are clearlydistinguishable without further image enhancement, showing the advantageof frame subtraction in accordance with this textured pattern sensingsystem implementation 700.

Other embodiments, uses and advantages of the inventive concept will beapparent to those skilled in the art from consideration of the abovedisclosure and the following claims. The specification should beconsidered non-limiting and exemplary only, and the scope of theinventive concept is accordingly intended to be limited only by thescope of the following claims.

1. A system for imaging a textured surface comprising: a light sourceconfigured to project electromagnetic radiation onto the texturedsurface, wherein the projected beam generates first radiation reflectedfrom a first portion of the textured surface to form a speckle pattern,and second radiation reflected from a second portion of the texturedsurface which is substantially uniform in intensity; and an opticaldetector configured to receive the first and second reflected radiationfrom the textured surface.
 2. The system according to claim 1, whereinthe light source is a pulsed laser.
 3. The system according to claim 1,wherein the light source is a partially coherent laser.
 4. The systemaccording to claim 1, wherein the light source is configured to projectelectromagnetic radiation that is generally undetectable by a humanunder ambient conditions.
 5. The system according to claim 1, furthercomprising: a processor configured to generate an image of the texturedsurface from the first and second reflected radiation.
 6. The systemaccording to claim 1, further comprising: a second light sourceconfigured to project a second electromagnetic radiation beam onto thetextured surface, wherein the second projected beam generates thirdradiation reflected from the first portion of the textured surface toform a speckle pattern, and fourth radiation reflected from the secondportion of the textured surface which is substantially uniform inintensity pattern; and a second optical detector configured to receivethe third and fourth reflected radiation from the textured surface. 7.The system according to claim 6, further comprising: a processorconfigured to generate an image of the textured surface from the thirdand fourth reflected radiation.
 8. The system according to claim 7,wherein the processor is configured to combine an image of the first andsecond reflected radiation and the image of the third and fourthreflected radiation.
 9. The system according to claim 8, wherein theprocessor uses frame subtraction so as to substantially eliminateimagery features from combined image of said second portion of thetextured surface.
 10. The system according to claim 6, wherein thesecond electromagnetic radiation beam has a different wavelength thanthe first electromagnetic radiation beam.
 11. The system according toclaim 1, wherein a coherence length of the light source is greater thana depth of the first region and less than a depth of the second region,when imaged within a single pixel of the optical detector.
 12. Thesystem according to claim 6, wherein a coherence length of said firstand second light sources is greater than a depth of the first region andless than a depth of the second region, when imaged within a singlepixel of the optical detector.
 13. A method for imaging a texturedsurface, comprising: projecting a beam of electromagnetic radiation ontothe textured surface, wherein the projected beam generates firstradiation reflected from a first portion of the textured surface to forma speckle pattern, and second radiation reflected from as a secondportion of the textured surface which is substantially uniform inintensity; and receiving, with an optical detector, the first and secondreflected radiation from the textured surface.
 14. The method accordingto claim 12, further comprising generating the electromagnetic radiationwith a pulsed laser.
 15. The method according to claim 12, wherein theprojected electromagnetic radiation is a partially coherent laser beam.16. The method according to claim 12, wherein the projectedelectromagnetic radiation is generally undetectable by a human underambient conditions.
 17. The method according to claim 12, furthercomprising: processing the first and second reflected radiation, with aprocessor, to generate an image of the textured surface.
 18. The methodaccording to claim 12, further comprising: projecting a second beam ofelectromagnetic radiation onto the textured surface, wherein theprojected beam generates third radiation reflected from the firstportion of the textured surface to form a speckle pattern, and secondradiation reflected from the second portion of the textured surfacewhich is substantially uniform in intensity; and receiving, with asecond optical detector, the third and fourth reflected radiation fromthe textured surface.
 19. The method according to claim 17, furthercomprising: processing the third and fourth reflected radiation, with aprocessor, to generate an image.
 20. The method according to claim 18,wherein the processing comprising combining an image of the first andsecond reflected radiation and the image of the third and fourthreflected radiation.
 21. The method according to claim 19, wherein theprocessing comprises frame subtraction so as to substantially eliminateimagery features from combined image of said second portion of thetextured surface.
 22. The system according to claim 17, wherein thesecond electromagnetic radiation beam has a different wavelength thanthe first electromagnetic radiation beam.
 23. The method according toclaim 12, wherein a coherence length of the light source is greater thana depth of the first region and less than a depth of the second regionwhen imaged within a single pixel of the optical detector.